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I would have to say my lighting went from pc to t5 and everything likes it. Also have been working on me water chemistry as well checking more things then I normally did when I first started in the hobby.

I would say my ATI PM 6 bulbs for my 75gal, as far as I remember of all times.
It's just amazing how much I've got out of that investment.
The zoas are doing much better with the reflectors and the choices of bulbs to change in spectrum is amazing.

Developing a rock solid maintenance routine with high quality water. I went from plagued with Cyano and bad qater quality to a tank that everything is doing great now.

Lots of great replies/info in this thread, but this one is by far my favortite. Maintenance, whether it is daily, weekly or monthly, can make the difference between success or tank/polyp failure. I think equally important is maintaining a log of everything you do, change, add, even visual changes within your system is so important.

I just added a skimmer to my 125g, 20g cube, 16g fragtank AIO. I thought having a big fuge would be enough. All my parameters were in check, but my sps showed slow growth. After added a skimmer, polyp extension increase and growth has gotten better. They're must have been a lot of DOC's in the water.